Drive control device for hybrid vehicle

ABSTRACT

A drive control device for a hybrid vehicle is provided with a differential device including four rotary elements; and an engine, first and second electric motors and an output rotary member respectively connected to the four rotary elements. One of the four rotary elements is constituted by a rotary component of a first differential mechanism and a rotary component of a second differential mechanism selectively connected through a clutch, and one of the rotary components is selectively fixed to a stationary member through a brake. The drive control device releases one of the clutch and the brake when the engine is started in a drive mode in which the engine is at rest while the clutch and the brake are in an engaged state, selecting the one of the clutch and the brake to be released, depending upon a drive force generated during starting the engine.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2012/057150, Mar. 21, 2012, the contents of all of which areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to an improvement of a drive controldevice for a hybrid vehicle.

BACKGROUND ART

There is known a hybrid vehicle which has at least one electric motor inaddition to an engine such as an internal combustion engine, whichfunctions as a vehicle drive power source. Patent Document 1 disclosesan example of such a hybrid vehicle, which is provided with an internalcombustion engine, a first electric motor and a second electric motor.This hybrid vehicle is further provided with a brake which is configuredto fix an output shaft of the above-described internal combustion engineto a stationary member, and an operating state of which is controlledaccording to a running condition of the hybrid vehicle, so as to improveenergy efficiency of the hybrid vehicle and to permit the hybrid vehicleto run according to a requirement by an operator of the hybrid vehicle.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: JP-2008-265600 A1

SUMMARY OF THE INVENTION Object Achieved by the Invention

However, the conventional arrangement of the hybrid vehicle describedabove has a risk that an operator of the hybrid vehicle feels uneasyabout generation of a reverse drive force, that is, a drive force(acceleration) in a direction opposite to the direction of running ofthe hybrid vehicle, upon starting of the engine while the hybrid vehicleis placed in a drive mode in which a vehicle drive force is generatedprimarily by the electric motor, with the engine being held at rest, forexample. The conventional arrangement does not make it possible tosufficiently reduce the uneasiness felt by the hybrid vehicle operatorupon starting of the engine in the above-indicated drive mode. Thisproblem was first discovered by the present inventors in the process ofintensive studies in an attempt to improve the performance of the hybridvehicle.

The present invention was made in view of the background art describedabove. It is therefore an object of the present invention to provide adrive control device for a hybrid vehicle, which permits reduction ofthe uneasiness felt by the hybrid vehicle operator upon starting of theengine which has been held at rest.

Means for Achieving the Object

The object indicated above is achieved according to a first aspect ofthe present invention, which provides a chive control device for ahybrid vehicle provided with: a first differential mechanism and asecond differential mechanism which have four rotary elements as awhole; and an engine, a first electric motor, a second electric motorand an output rotary member which are respectively connected to theabove-described four rotary elements, and wherein one of theabove-described four rotary elements is constituted by the rotaryelement of the above-described first differential mechanism and therotary element of the above-described second differential mechanismwhich are selectively connected to each other through a clutch, and oneof the rotary elements of the above-described first and seconddifferential mechanisms which are selectively connected to each otherthrough the above-described clutch is selectively fixed to a stationarymember through a brake, the above-described drive control device beingcharacterized by releasing at least one of the above-described clutchand the above-described brake when the above-described engine is startedin a drive mode in which the above-described engine is held at restwhile the above-described clutch and the above-described brake are bothplaced in an engaged state.

Advantages of the Invention

According to the first aspect of the invention described above, thehybrid vehicle is provided with: the first differential mechanism andthe second differential mechanism which have the four rotary elements asa whole; and the engine, the first electric motor, the second electricmotor and the output rotary member which are respectively connected tothe four rotary elements. One of the above-described four rotaryelements is constituted by the rotary element of the above-describedfirst differential mechanism and the rotary element of theabove-described second differential mechanism which are selectivelyconnected to each other through the clutch, and one of the rotaryelements of the above-described first and second differential mechanismswhich are selectively connected to each other through the clutch isselectively fixed to the stationary member through the brake.Accordingly, it is possible to effectively reduce the generation of thereverse drive force upon starting of the engine which has been held atrest. Namely, the present invention provides the drive control devicefor the hybrid vehicle, which permits reduction of uneasiness felt bythe hybrid vehicle operator upon starting of the engine which has beenheld at rest.

According to a second aspect of the invention, the drive control deviceaccording to the first aspect of the invention is configured to releaseone of the above-described clutch and the above-described brake when theabove-described engine is started in the above-indicated drive mode, andto select the above-indicated one of the clutch and the brake which isto be released, depending upon a drive force generated to drive theabove-described hybrid vehicle during starting of the above-describedengine. According to this second aspect of the invention, it is possibleto effectively and practically reduce the generation of the reversedrive force upon starting of the engine which has been held at rest.

According to a third aspect of the invention, the drive control deviceaccording to the first or second aspect of the invention is configuredto release one of the above-described clutch and the above-describedbrake when the above-described engine is started in the above-indicateddrive mode, and to release the brake when it is determined that anoperator of the hybrid vehicle is unlikely to feel generation of areverse drive force due to starting of the engine. According to thisthird aspect of the invention, it is possible to effectively andpractically reduce the generation of the reverse drive force uponstarting of the engine which has been held at rest.

According to a fourth aspect of the invention, the drive control deviceaccording to the second aspect of the invention is configured to releasethe above-described brake when the above-indicated drive force is equalto or larger than a predetermined threshold value. According to thisfourth aspect of the invention, it is possible to effectively andpractically reduce the generation of the reverse drive force uponstarting of the engine which has been held at rest.

According to a fifth aspect of the invention, the drive control deviceaccording to the second aspect of the invention is configured to releasethe clutch when the above-indicated drive force is smaller than apredetermined threshold value. According to this fifth aspect of theinvention, it is possible to effectively and practically reduce thegeneration of the reverse drive force upon starting of the engine whichhas been held at rest.

According to a sixth aspect of the invention, the drive control deviceaccording to the first or second aspect of the invention, or accordingto the third aspect of the invention according to the first or secondaspect of the invention, or according to the fourth or fifth aspect ofthe invention is configured such that the above-described firstdifferential mechanism is provided with a first rotary element connectedto the above-described first electric motor, a second rotary elementconnected to the above-described engine, and a third rotary elementconnected to the above-described output rotary member, while theabove-described second differential mechanism is provided with a firstrotary element connected to the above-described second electric motor, asecond rotary element, and a third rotary element, one of the second andthird rotary elements of the second differential mechanism beingconnected to the third rotary element of the above-described firstdifferential mechanism, and wherein the above-described clutch isconfigured to selectively connect the second rotary element of theabove-described first differential mechanism, and the other of thesecond and third rotary elements of the above-described seconddifferential mechanism which is not connected to the third rotaryelement of the above-described first differential mechanism, to eachother, while the above-described brake is configured to selectively fixthe other of the second and third rotary elements of the above-describedsecond differential mechanism which is not connected to the third rotaryelement of the above-described first differential mechanism, to thestationary member. According to this sixth aspect of the invention, itis possible to reduce the uneasiness felt by the operator of the hybridvehicle upon starting of the engine which has been held at rest, in ahybrid vehicle drive system having a highly practical arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view for explaining an arrangement of a hybridvehicle drive system to which the present invention is suitablyapplicable;

FIG. 2 is a view for explaining major portions of a control systemprovided to control the drive system of FIG. 1;

FIG. 3 is a table indicating combinations of operating states of aclutch and a brake, which correspond to respective five drive modes ofthe drive system of FIG. 1;

FIG. 4 is a collinear chart having straight lines which permitindication thereon of relative rotating speeds of various rotaryelements of the drive system of FIG. 1, the collinear chartcorresponding to the modes 1 and 3 of FIG. 3;

FIG. 5 is a collinear chart having straight lines which permitindication thereon of relative rotating speeds of various rotaryelements of the drive system of FIG. 1, the collinear chartcorresponding to the mode 2 of FIG. 3;

FIG. 6 is a collinear chart having straight lines which permitindication thereon of relative rotating speeds of various rotaryelements of the drive system of FIG. 1, the collinear chartcorresponding to the mode 4 of FIG. 3;

FIG. 7 is a collinear chart having straight lines which permitindication thereon of relative rotating speeds of various rotaryelements of the drive system of FIG. 1, the collinear chartcorresponding to the mode 5 of FIG. 3;

FIG. 8 is a view for explaining transmission efficiency of the drivesystem of FIG. 1;

FIG. 9 is a functional block diagram for explaining major controlfunctions of an electronic control device provided for the drive systemof FIG. 1;

FIG. 10 is a flow chart for explaining an example of an engine startingcontrol implemented by the electronic control device provided for thedrive system of FIG. 1;

FIG. 11 is a flow chart for explaining another example of the enginestarting control implemented by the electronic control device providedfor the drive system of FIG. 1;

FIG. 12 is a schematic view for explaining an arrangement of a hybridvehicle drive system according to another preferred mode of thisinvention;

FIG. 13 is a schematic view for explaining an arrangement of a hybridvehicle drive system according to a further preferred mode of thisinvention;

FIG. 14 is a schematic view for explaining an arrangement of a hybridvehicle drive system according to a still further preferred mode of thisinvention;

FIG. 15 is a schematic view for explaining an arrangement of a hybridvehicle drive system according to a yet further preferred mode of thisinvention;

FIG. 16 is a schematic view for explaining an arrangement of a hybridvehicle drive system according to still another preferred mode of thisinvention;

FIG. 17 is a schematic view for explaining an arrangement of a hybridvehicle drive system according to yet another preferred mode of thisinvention;

FIG. 18 is a collinear chart for explaining an arrangement and anoperation of a hybrid vehicle drive system according to anotherpreferred mode of this invention;

FIG. 19 is a collinear chart for explaining an arrangement and anoperation of a hybrid vehicle drive system according to a furtherpreferred mode of this invention; and

FIG. 20 is a collinear chart for explaining an arrangement and anoperation of a hybrid vehicle drive system according to a still furtherpreferred mode of this invention.

MODE FOR CARRYING OUT THE INVENTION

According to the present invention, the first and second differentialmechanisms as a whole have four rotary elements while theabove-described clutch is placed in the engaged state. In one preferredform of the present invention, the first and second differentialmechanism as a whole have four rotary elements while a plurality ofclutches, each of which is provided between the rotary elements of thefirst and second differential mechanisms and which includes theabove-described clutch, are placed in their engaged states. In otherwords, the present invention is suitably applicable to a drive controldevice for a hybrid vehicle which is provided with the first and seconddifferential mechanisms represented as the four rotary elementsindicated in a collinear chart, the engine, the first electric motor,the second electric motor and the output rotary member coupled to therespective four rotary elements, and wherein one of the four rotaryelements is selectively connected through the above-described clutch toanother of the rotary elements of the first differential mechanism andanother of the rotary elements of the second differential mechanism,while the rotary element of the first or second differential mechanismto be selectively connected to the above-indicated one rotary elementthrough the clutch is selectively fixed through the above-describedbrake to the stationary member.

In another preferred form of the present invention, the above-describedclutch and brake are hydraulically operated coupling devices operatingstates (engaged and released states) of which are controlled accordingto a hydraulic pressure. While wet multiple-disc type frictionalcoupling devices are preferably used as the clutch and brake, meshingtype coupling devices, namely, so-called dog clutches (claw clutches)may also be used. Alternatively, the clutch and brake may beelectromagnetic clutches, magnetic powder clutches and any otherclutches the operating states of which are controlled (which are engagedand released) according to electric commands.

The drive system to which the present invention is applicable is placedin a selected one of a plurality of drive modes, depending upon theoperating states of the above-described clutch and brake. Preferably, EVdrive modes in which at least one of the above-described first andsecond electric motors is used as a vehicle drive power source with theengine stopped include a mode 1 to be established in the engaged stateof the brake and in the released state of the clutch, and a mode 2 to beestablished in the engaged states of both of the clutch and brake.Further, hybrid drive modes in which the above-described engine isoperated while the above-described first and second electric motors areoperated to generate a vehicle drive force and/or an electric energy asneeded, include a mode 3 to be established in the engaged state of thebrake and in the released state of the clutch, a mode 4 to beestablished in the released state of the brake and the engaged state ofthe clutch, and a mode 5 to be established in the released states ofboth of the brake and clutch.

In a further preferred form of the invention, the rotary elements of theabove-described first differential mechanism, and the rotary elements ofthe above-described second differential mechanism are arranged as seenin the collinear charts, in the engaged state of the above-describedclutch and in the released state of the above-described brake, in theorder of the first rotary element of the first differential mechanism,the first rotary element of the second differential mechanism, thesecond rotary element of the first differential mechanism, the secondrotary element of the second differential mechanism, the third rotaryelement of the first differential mechanism, and the third rotaryelement of the second differential mechanism, where the rotating speedsof the second rotary elements and the third rotary elements of the firstand second differential mechanisms are indicated in mutually overlappingstates in the collinear charts.

In a still further preferred form of the invention, it is determinedthat the operator of the hybrid vehicle is unlikely to feel generationof a reverse drive force upon starting of the engine during which adrive force to drive the hybrid vehicle is generated, if the generateddrive force is equal to or larger than a predetermined threshold value.Preferably, the determination that the operator of the hybrid vehicle isunlikely to feel the generation of the reverse drive force upon startingof the engine is made when a vehicle braking operation is performed(when a braking device is placed in an operated state). More preferably,this determination is made when the vehicle braking operation isperformed and the running speed of the hybrid vehicle is zero.Preferably, the determination that the operator of the hybrid vehicle isunlikely to feel the generation of the reverse drive force upon startingof the engine is made, when a manually operated shifting device isplaced in a parking position, that is, a “P” position.

Referring to the drawings, preferred embodiments of the presentinvention will be described in detail. It is to be understood that thedrawings referred to below do not necessarily accurately representratios of dimensions of various elements.

First Embodiment

FIG. 1 is the schematic view for explaining an arrangement of a hybridvehicle drive system 10 (hereinafter referred to simply as a “drivesystem 10”) to which the present invention is suitably applicable. Asshown in FIG. 1, the drive system 10 according to the present embodimentis of a transversely installed type suitably used for an FF(front-engine front-drive) type vehicle, and is provided with a mainvehicle drive power source in the form of an engine 12, a first electricmotor MG1, a second electric motor MG2, a first differential mechanismin the form of a first planetary gear set 14, and a second differentialmechanism in the form of a second planetary gear set 16, which aredisposed on a common center axis CE. The drive system 10 is constructedsubstantially symmetrically with respect to the center axis CE. In FIG.1, a lower half of the drive system 10 is not shown. This descriptionapplies to other embodiments which will be described.

The engine 12 is an internal combustion engine such as a gasolineengine, which is operable to generate a drive force by combustion of afuel such as a gasoline injected into its cylinders. Each of the firstelectric motor MG1 and second electric motor MG2 is a so-calledmotor/generator having a function of a motor operable to generate adrive force, and a function of an electric generator operable togenerate a reaction force, and is provided with a stator 18, 22 fixed toa stationary member in the form of a housing (casing) 26, and a rotor20, 24 disposed radially inwardly of the stator 18, 22.

The first planetary gear set 14 is a single-pinion type planetary gearset which has a gear ratio ρ1 and which is provided with rotary elements(elements) consisting of: a first rotary element in the form of a sungear S1; a second rotary element in the form of a carrier C1 supportinga pinion gear P1 such that the pinion gear P1 is rotatable about itsaxis and the axis of the planetary gear set; and a third rotary elementin the form of a ring gear R1 meshing with the sun gear S1 through thepinion gear P1. The second planetary gear set 16 is a single-pinion typeplanetary gear set which has a gear ratio ρ2 and which is provided withrotary elements (elements) consisting of: a first rotary element in theform of a sun gear S2; a second rotary element in the form of a carrierC2 supporting a pinion gear P2 such that the pinion gear P2 is rotatableabout its axis and the axis of the planetary gear set; and a thirdrotary element in the form of a ring gear R2 meshing with the sun gearS2 through the pinion gear P2.

The sun gear S1 of the first planetary gear set 14 is connected to therotor 20 of the first electric motor MG1. The carrier C1 of the firstplanetary gear set 14 is connected to an input shaft 28 which is rotatedintegrally with a crankshaft of the engine 12. This input shaft 28 isrotated about the center axis CE. In the following description, thedirection of extension of this center axis CE will be referred to as an“axial direction”, unless otherwise specified. The ring gear R1 of thefirst planetary gear set 14 is connected to an output rotary member inthe form of an output gear 30, and to the ring gear R2 of the secondplanetary gear set 16. The sun gear S2 of the second planetary gear set16 is connected to the rotor 24 of the second electric motor MG2.

The drive force received by the output gear 30 is transmitted to a pairof left and right drive wheels (not shown) through a differential geardevice not shown and axles not shown. On the other hand, a torquereceived by the drive wheels from a roadway surface on which the vehicleis running is transmitted (input) to the output gear 30 through thedifferential gear device and axles, and to the drive system 10. Amechanical oil pump 32, which is a vane pump, for instance, is connectedto one of opposite end portions of the input shaft 28, which one endportion is remote from the engine 12. The oil pump 32 is operated by theengine 12, to generate a hydraulic pressure to be applied to a hydrauliccontrol unit 60, etc. which will be described. An electrically operatedoil pump which is operated with an electric energy may be provided inaddition to the oil pump 32.

Between the carrier C1 of the first planetary gear set 14 and thecarrier C2 of the second planetary gear set 16, there is disposed aclutch CL which is configured to selectively couple these carriers C1and C2 to each other (to selectively connect the carriers C1 and C2 toeach other or disconnect the carriers C1 and C2 from each other).Between the carrier C2 of the second planetary gear set 16 and thestationary member in the form of the housing 26, there is disposed abrake BK which is configured to selectively couple (fix) the carrier C2to the housing 26. Each of these clutch CL and brake BK is ahydraulically operated coupling device the operating state of which iscontrolled (which is engaged and released) according to the hydraulicpressure applied thereto from the hydraulic control unit 60. While wetmultiple-disc type frictional coupling devices are preferably used asthe clutch CL and brake BK, meshing type coupling devices, namely,so-called dog clutches (claw clutches) may also be used. Alternatively,the clutch CL and brake BK may be electromagnetic clutches, magneticpowder clutches and any other clutches the operating states of which arecontrolled (which are engaged and released) according to electriccommands generated from an electronic control device 40.

As shown in FIG. 1, the drive system 10 is configured such that thefirst planetary gear set 14 and second planetary gear set 16 aredisposed coaxially with the input shaft 28 (disposed on the center axisCE), and opposed to each other in the axial direction of the center axisCE. Namely, the first planetary gear set 14 is disposed on one side ofthe second planetary gear set 16 on a side of the engine 12, in theaxial direction of the center axis CE. The first electric motor MG1 isdisposed on one side of the first planetary gear set 14 on the side ofthe engine 12, in the axial direction of the center axis CE. The secondelectric motor MG2 is disposed on one side of the second planetary gearset 16 which is remote from the engine 12, in the axial direction of thecenter axis CE. Namely, the first electric motor MG1 and second electricmotor MG2 are opposed to each other in the axial direction of the centeraxis CE, such that the first planetary gear set 14 and second planetarygear set 16 are interposed between the first electric motor MG1 andsecond electric motor MG2. That is, the drive system 10 is configuredsuch that the first electric motor MG1, first planetary gear set 14,clutch CL, second planetary gear set 16, brake BK and second electricmotor MG2 are disposed coaxially with each other, in the order ofdescription from the side of the engine 12, in the axial direction ofthe center axis CE.

FIG. 2 is the view for explaining major portions of a control systemprovided to control the drive system 10. The electronic control device40 shown in FIG. 2 is a so-called microcomputer which incorporates aCPU, a ROM, a RAM and an input-output interface and which is operable toperform signal processing operations according to programs stored in theROM while utilizing a temporary data storage function of the RAM, toimplement various drive controls of the drive system 10, such as a drivecontrol of the engine 12 and hybrid drive controls of the first electricmotor MG1 and second electric motor MG2. In the present embodiment, theelectronic control device 40 corresponds to a drive control device for ahybrid vehicle having the drive system 10. The electronic control device40 may be constituted by mutually independent control units as neededfor respective controls such as an output control of the engine 12 anddrive controls of the first electric motor MG1 and second electric motorMG2.

As indicated in FIG. 2, the electronic control device 40 is configuredto receive various signals from sensors and switches provided in thedrive system 10. Namely, the electronic control device 40 receives: anoutput signal of an accelerator pedal operation amount sensor 42indicative of an operation amount or angle A_(CC) of an acceleratorpedal (not shown), which corresponds to a vehicle output required by avehicle operator; an output signal of an engine speed sensor 44indicative of an engine speed N_(E), that is, an operating speed of theengine 12; an output signal of an MG1 speed sensor 46 indicative of anoperating speed N_(MG1) of the first electric motor MG1; an outputsignal of an MG2 speed sensor 48 indicative of an operating speedN_(MG2) of the second electric motor MG2; an output signal of an outputspeed sensor 50 indicative of a rotating speed N_(OUT) of the outputgear 30, which corresponds to a running speed V of the vehicle; anoutput signal of a shift position sensor 52 indicative of a presentlyselected operating position (shift position) P_(S) of a manuallyoperated shifting device not shown; an output signal of a battery SOCsensor 54 indicative of stored electric energy amount (state of charge)SOC of a battery not shown; an output signal of a brake sensor 55indicative of an operated or non-operated position of a manuallyoperable braking member (foot brake pedal) not shown.

The electronic control device 40 is also configured to generate variouscontrol commands to be applied to various portions of the drive system10. Namely, the electronic control device 40 applies to an enginecontrol device 56 for controlling an output of the engine 12, followingengine output control commands for controlling the output of the engine12, which commands include: a fuel injection amount control signal tocontrol an amount of injection of a fuel by a fuel injecting device intoan intake pipe; an ignition control signal to control a timing ofignition of the engine 12 by an igniting device; and an electronicthrottle valve drive control signal to control a throttle actuator forcontrolling an opening angle θ_(TH) of an electronic throttle valve.Further, the electronic control device 40 applies command signals to aninverter 58, for controlling operations of the first electric motor MG1and second electric motor MG2, so that the first and second electricmotors MG1 and MG2 are operated with electric energies supplied theretofrom a battery through the inverter 58 according to the command signalsto control outputs (output torques) of the electric motors MG1 and MG2.Electric energies generated by the first and second electric motors MG1and MG2 are supplied to and stored in the battery through the inverter58. Further, the electronic control device 40 applies command signalsfor controlling the operating states of the clutch CL and brake BK, tolinear solenoid valves and other electromagnetic control valves providedin the hydraulic control unit 60, so that hydraulic pressures generatedby those electromagnetic control valves are controlled to control theoperating states of the clutch CL and brake BK.

An operating state of the drive system 10 is controlled through thefirst electric motor MG1 and second electric motor MG2, such that thedrive system 10 functions as an electrically controlled differentialportion whose difference of input and output speeds is controllable. Forexample, an electric energy generated by the first electric motor MG1 issupplied to the battery or the second electric motor MG2 through theinverter 58. Namely, a major portion of the drive force of the engine 12is mechanically transmitted to the output gear 30, while the remainingportion of the drive force is consumed by the first electric motor MG1operating as the electric generator, and converted into the electricenergy, which is supplied to the second electric motor MG2 through theinverter 58, so that the second electric motor MG2 is operated togenerate a drive force to be transmitted to the output gear 30.Components associated with the generation of the electric energy and theconsumption of the generated electric energy by the second electricmotor MG2 constitute an electric path through which a portion of thedrive force of the engine 12 is converted into an electric energy whichis converted into a mechanical energy.

In the hybrid vehicle provided with the drive system 10 constructed asdescribed above, one of a plurality of drive modes is selectivelyestablished according to the operating states of the engine 12, firstelectric motor MG1 and second electric motor MG2, and the operatingstates of the clutch CL and brake BK. FIG. 3 is the table indicatingcombinations of the operating states of the clutch CL and brake BK,which correspond to the respective five drive modes of the drive system10. In this table, “o” marks represent an engaged state while blanksrepresent a released state. The drive modes EV-1 and EV-2 indicated inFIG. 3 are EV drive modes in which the engine 12 is held at rest whileat least one of the first electric motor MG1 and second electric motorMG2 is used as a vehicle drive power source. The drive modes HV-1 , HV-2and HV-3 are hybrid drive modes (HV modes) in which the engine 12 isoperated as the vehicle drive power source while the first electricmotor MG1 and second electric motor MG2 are operated as needed togenerate a vehicle drive force and/or an electric energy. In thesehybrid drive modes, at least one of the first electric motor MG1 andsecond electric motor MG2 is operated to generate a reaction force orplaced in a non-load free state.

As is apparent from FIG. 3, the EV drive modes of the drive system 10 inwhich the engine 12 is held at rest while at least one of the firstelectric motor MG1 and second electric motor MG2 is used as the vehicledrive power source consist of: a mode 1 (drive mode 1) in the form ofthe drive mode EV 1 which is established in the engaged state of thebrake BK and in the released state of the clutch CL; and a mode 2 (drivemode 2) in the form of the drive mode EV-2 which is established in theengaged states of both of the brake BK and clutch CL. The hybrid drivemodes in which the engine 12 is operated as the vehicle drive powersource while the first electric motor MG1 and second electric motor MG2are operated as needed to generate a vehicle drive force and/or anelectric energy, consist of: a mode 3 (drive mode 3) in the form of thedrive mode HV-1 which is established in the engaged state of the brakeBK and in the released state of the clutch CL; a mode 4 (drive mode 4)in the form of the drive mode HV-2 which is established in the releasedstate of the brake BK and in the engaged state of the clutch CL; and amode 5 (drive mode 5) in the form of the drive mode HV-3 which isestablished in the released states of both of the brake BK and clutchCL.

FIGS. 4-7 are the collinear charts having straight lines which permitindication thereon of relative rotating speeds of the various rotaryelements of the drive system 10 (first planetary gear set 14 and secondplanetary gear set 16), which rotary elements are connected to eachother in different manners corresponding to respective combinations ofthe operating states of the clutch CL and brake BK. These collinearcharts are defined in a two-dimensional coordinate system having ahorizontal axis along which relative gear ratios ρ of the first andsecond planetary gear sets 14 and 16 are taken, and a vertical axisalong which the relative rotating speeds are taken. The collinear chartsindicate the relative rotating speeds when the output gear 30 is rotatedin the positive direction to drive the hybrid vehicle in the forwarddirection. A horizontal line X1 represents the rotating speed of zero,while vertical lines Y1 through Y4 arranged in the order of descriptionin the rightward direction represent the respective relative rotatingspeeds of the sun gear S1, sun gear S2, carrier C1 and ring gear R1.Namely, a solid line Y1 represents the relative rotating speed of thesun gear S1 of the first planetary gear set 14 (operating speed of thefirst electric motor MG1), a broken line Y2 represents the relativerotating speed of the sun gear S2 of the second planetary gear set 16(operating speed of the second electric motor MG2), a solid line Y3represents the relative rotating speed of the carrier C1 of the firstplanetary gear set 14 (operating speed of the engine 12), a broken lineY3′ represents the relative rotating speed of the carrier C2 of thesecond planetary gear set 16, a solid line Y4 represents the relativerotating speed of the ring gear R1 of the first planetary gear set 14(rotating speed of the output gear 30), and a broken line Y4′ representsthe relative rotating speed of the ring gear R2 of the second planetarygear set 16. In FIGS. 4-7, the vertical lines Y3 and Y3′ aresuperimposed on each other, while the vertical lines Y4 and Y4′ aresuperimposed on each other. Since the ring gears R1 and R2 are fixed toeach other, the relative rotating speeds of the ring gears R1 and R2represented by the vertical lines Y4 and Y4′ are equal to each other.

In FIGS. 4-7, a solid line L1 represents the relative rotating speeds ofthe three rotary elements of the first planetary gear set 14, while abroken line L2 represents the relative rotating speeds of the threerotary elements of the second planetary gear set 16. Distances betweenthe vertical lines Y1-Y4 (Y2-Y4′) are determined by the gear ratios ρ1and ρ2 of the first and second planetary gear sets 14 and 16. Describedmore specifically, regarding the vertical lines Y1 , Y3 and Y4corresponding to the respective three rotary elements in the form of thesun gear S1, carrier C1 and ring gear R1 of the first planetary gear set14, a distance between the vertical lines Y1 and Y3 corresponds to “1”,while a distance between the vertical lines Y3 and Y4 corresponds to thegear ratio “ρ1”. Regarding the vertical lines Y2, Y3′ and Y4′corresponding to the respective three rotary elements in the form of thesun gear S2, carrier C2 and ring gear R2 of the second planetary gearset 16, a distance between the vertical lines Y2 and Y3′ corresponds to“1”, while a distance between the vertical lines Y3′ and Y4′ correspondsto the gear ratio “ρ2”. In the drive system 10, the gear ratio ρ2 of thesecond planetary gear set 16 is higher than the gear ratio ρ1 of thefirst planetary gear set 14 (ρ2>ρ1). The drive modes of the drive system10 will be described by reference to FIGS. 4-7.

The drive mode EV-1 indicated in FIG. 3 corresponds to the mode 1 (drivemode 1) of the drive system 10, which is preferably the EV drive mode inwhich the engine 12 is held at rest while the second electric motor MG2is used as the vehicle drive power source. FIG. 4 is the collinear chartcorresponding to the mode 1. Described by reference to this collinearchart, the carrier C1 of the first planetary gear set 14 and the carrierC2 of the second planetary gear set 16 are rotatable relative to eachother in the released state of the clutch CL. In the engaged state ofthe brake BK, the carrier C2 of the second planetary gear set 16 iscoupled (fixed) to the stationary member in the form of the housing 26,so that the rotating speed of the carrier C2 is held zero. In this mode1, the rotating direction of the sun gear S2 and the rotating directionof the ring gear R2 in the second planetary gear set 16 are opposite toeach other, so that when the second electric motor MG2 is operated togenerate a negative torque (acting in the negative direction), the ringgear R2, that is, the output gear 30 is rotated in the positivedirection by the generated negative torque. Namely, the hybrid vehicleprovided with the drive system 10 is driven in the forward directionwhen the negative torque is generated by the second electric motor MG2.In this case, the first electric motor MG1 is preferably held in a freestate. In this mode 1, the carriers C1 and C2 are permitted to berotated relative to each other, so that the hybrid vehicle can be drivenin the EV drive mode similar to an EV drive mode which is established ina vehicle provided with a so-called “THS” (Toyota Hybrid System) and inwhich the carrier C2 is fixed to the stationary member.

The drive mode EV-2 indicated in FIG. 3 corresponds to the mode 2 (drivemode 2) of the drive system 10, which is preferably the EV drive mode inwhich the engine 12 is held at rest while at least one of the firstelectric motor MG1 and second electric motor MG2 is used as the vehicledrive power source. FIG. 5 is the collinear chart corresponding to themode 2. Described by reference to this collinear chart, the carrier C1of the first planetary gear set 14 and the carrier C2 of the secondplanetary gear set 16 are not rotatable relative to each other in theengaged state of the clutch CL. Further, in the engaged state of thebrake BK, the carrier C2 of the second planetary gear set 16 and thecarrier C1 of the first planetary gear set 14 which is connected to thecarrier C2 are coupled (fixed) to the stationary member in the form ofthe housing 26, so that the rotating speeds of the carriers C1 and C2are held zero. In this mode 2, the rotating direction of the sun gear S1and the rotating direction of the ring gear R1 in the first planetarygear set 14 are opposite to each other, and the rotating direction ofthe sun gear S2 and the rotating direction of the ring gear R2 in thesecond planetary gear set 16 are opposite to each other, so that whenthe first electric motor MG1 and/or second electric motor MG2 is/areoperated to generate a negative torque (acting in the negativedirection), the ring gears R1 and R2 are rotated, that is, the outputgear 30 is rotated in the positive direction by the generated negativetorque. Namely, the hybrid vehicle provided with the drive system 10 isdriven in the forward direction when the negative torque is generated byat least one of the first electric motor MG1 and second electric motorMG2.

In the mode 2, at least one of the first electric motor MG1 and secondelectric motor MG2 may be operated as the electric generator. In thiscase, one or both of the first and second electric motors MG1 and MG2may be operated to generate a vehicle drive force (torque), at anoperating point assuring a relatively high degree of operatingefficiency, and/or with a reduced degree of torque limitation due toheat generation. Further, at least one of the first and second electricmotors MG1 and MG2 may be held in a free state, when the generation ofan electric energy by a regenerative operation of the electric motorsMG1 and MG2 is inhibited due to full charging of the battery. Namely,the mode 2 is an EV drive mode in which amounts of work to be assignedto the first and second electric motors MG1 and MG2 can be adjusted withrespect to each other, and which may be established under variousrunning conditions of the hybrid vehicle, or may be kept for arelatively long length of time. Accordingly, the mode 2 isadvantageously provided on a hybrid vehicle such as a plug-in hybridvehicle, which is frequently placed in an EV drive mode.

The drive mode HV-1 indicated in FIG. 3 corresponds to the mode 3 (drivemode 3) of the drive system 10, which is preferably the HV drive mode inwhich the engine 12 is used as the vehicle drive power source while thefirst electric motor MG1 and second electric motor MG2 are operated asneeded to generate a vehicle drive force and/or an electric energy. FIG.4 is the collinear chart corresponding to the mode 3. Described byreference to this collinear chart, the carrier C1 of the first planetarygear set 14 and the carrier C2 of the second planetary gear set 16 arerotatable relative to each other, in the released state of the clutchCL. In the engaged state of the brake BK, the carrier C2 of the secondplanetary gear set 16 is coupled (fixed) to the stationary member in theform of the housing 26, so that the rotating speed of the carrier C2 isheld zero. In this mode 3 , the engine 12 is operated to generate anoutput torque by which the output gear 30 is rotated. At this time, thefirst electric motor MG1 is operated to generate a reaction torque inthe first planetary gear set 14, so that the output of the engine 12 canbe transmitted to the output gear 30. In the second planetary gear set16, the rotating direction of the sun gear S2 and the rotating directionof the ring gear R2 are opposite to each other, in the engaged state ofthe brake BK, so that when the second electric motor MG2 is operated togenerate a negative torque (acting in the negative direction), the ringgears R1 and R2 are rotated, that is, the output gear 30 is rotated inthe positive direction by the generated negative torque.

The drive mode HV-2 indicated in FIG. 3 corresponds to the mode 4 (drivemode 4) of the drive system 10, which is preferably the HV drive mode inwhich the engine 12 is used as the vehicle drive power source while thefirst electric motor MG1 and second electric motor MG2 are operated asneeded to generate a vehicle drive force and/or an electric energy. FIG.6 is the collinear chart corresponding to the mode 4. Described byreference to this collinear chart, the carrier C1 of the first planetarygear set 14 and the carrier C2 of the second planetary gear set 16 arenot rotatable relative to each other, in the engaged state of the clutchCL, that is, the carriers C1 and C2 are integrally rotated as a singlerotary element. The ring gears R1 and R2, which are fixed to each other,are integrally rotated as a single rotary element. Namely, in the mode 4of the drive system 10, the first planetary gear set 14 and secondplanetary gear set 16 function as a differential mechanism having atotal of four rotary elements. That is, the drive mode 4 is a compositesplit mode in which the four rotary elements consisting of the sun gearS1 (connected to the first electric motor MG1), the sun gear S2(connected to the second electric motor MG2), the rotary elementconstituted by the carriers C1 and C2 connected to each other (and tothe engine 12), and the rotary element constituted by the ring gears R1and R2 fixed to each other (and connected to the output gear 30) areconnected to each other in the order of description in the rightwarddirection as seen in FIG. 6.

In the mode 4, the rotary elements of the first planetary gear set 14and second planetary gear set 16 are preferably arranged as indicated inthe collinear chart of FIG. 6, that is, in the order of the sun gear S1represented by the vertical line Y1 , the sun gear S2 represented by thevertical line Y2 , the carriers C1 and C2 represented by the verticalline Y3 (Y3′), and the ring gears R1 and R2 represented by the verticalline Y4 (Y4′). The gear ratios ρ1 and ρ2 of the first and secondplanetary gear sets 14 and 16 are determined such that the vertical lineY1 corresponding to the sun gear S1 and the vertical line Y2corresponding to the sun gear S2 are positioned as indicated in thecollinear chart of FIG. 6, namely, such that the distance between thevertical lines Y1 and Y3 is longer than the distance between thevertical lines Y2 and Y3′. In other words, the distance between thevertical lines corresponding to the sun gear S1 and the carrier C1 andthe distance between the vertical lines corresponding to the sun gear S2and the carrier C2 correspond to “1”, while the distance between thevertical lines corresponding to the carrier C1 and the ring gear R1 andthe distance between the vertical lines corresponding to the carrier C2and the ring gear R2 correspond to the respective gear ratios ρ1 and ρ2.Accordingly, the drive system 10 is configured such that the gear ratioρ2 f the second planetary gear set 16 is higher than the gear ratio ρ1of the first planetary gear set 14.

In the mode 4, the carrier C1 of the first planetary gear set 14 and thecarrier C2 of the second planetary gear set 16 are connected to eachother in the engaged state of the clutch CL, so that the carriers C1 andC2 are rotated integrally with each other. Accordingly, either one orboth of the first electric motor MG1 and second electric motor MG2 canreceive a reaction force corresponding to the output of the engine 12.Namely, one or both of the first and second electric motors MG1 and MG2can be operated to receive the reaction force during an operation of theengine 12, in other words, the amounts of work to be assigned to thefirst and second electric motors MG1 and MG2 can be adjusted withrespect to each other. That is, each of the first and second electricmotors MG1 and MG2 can be operated at an operating point assuring arelatively high degree of operating efficiency, and/or with a reduceddegree of torque limitation due to heat generation.

For example, one of the first electric motor MG1 and second electricmotor MG2 which is operable with a higher degree of operating efficiencyis preferentially operated to generate a reaction force, so that theoverall operating efficiency can be improved. When the hybrid vehicle isdriven at a comparatively high running speed V and at a comparativelylow engine speed N_(E), for instance, the operating speed N_(MG1) of thefirst electric motor MG1 may have a negative value, that is, the firstelectric motor MG1 may be operated in the negative direction. In thecase where the first electric motor MG1 generates the reaction forceacting on the engine 12, the first electric motor MG1 is operated in thenegative direction so as to generate a negative torque with consumptionof an electric energy, giving rise to a risk of reduction of theoperating efficiency. In this respect, it will be apparent from FIG. 6that in the drive system 10, the operating speed of the second electricmotor MG2 indicated on the vertical line Y2 is less likely to have anegative value than the operating speed of the above-indicated firstelectric motor MG1 indicated on the vertical line Y1 , and the secondelectric motor MG2 may possibly be operated in the positive direction,during generation of the reaction force. Accordingly, it is possible toimprove the operating efficiency to improve the fuel economy, bypreferentially controlling the second electric motor MG2 so as togenerate the reaction force, while the operating speed of the firstelectric motor MG1 has a negative value. Further, where there is atorque limitation of one of the first electric motor MG1 and secondelectric motor MG2 due to heat generation, it is possible to ensure thegeneration of the reaction force required for the engine 12, bycontrolling the other electric motor so as to perform a regenerativeoperation or a vehicle driving operation, for providing an assistingvehicle driving force.

FIG. 8 is the view for explaining transmission efficiency of the drivesystem 10, wherein a speed ratio is taken along the horizontal axiswhile theoretical transmission efficiency is taken along the verticalaxis. The speed ratio indicated in FIG. 8 is a ratio of the input sidespeed of the first and second planetary gear sets 14 and 16 to theoutput side speed, that is, the speed reduction ratio, which is forexample, a ratio of the rotating speed of the input rotary member in theform of the carrier C1 to the rotating speed of the output gear 30 (ringgears R1 and R2). The speed ratio is taken along the horizontal axis inFIG. 8 such that the left side as seen in the view of FIG. 8 is a sideof high gear positions having comparatively low speed ratio values whilethe right side is a side of low gear positions having comparatively highspeed ratio values. Theoretical transmission efficiency indicated inFIG. 8 is a theoretical value of the transmission efficiency of thedrive system 10, which has a maximum value of 1.0 when an entirety ofthe drive force is mechanically transmitted from the first and secondplanetary gear sets 14 and 16 to the output gear 30, withouttransmission of an electric energy through the electric path.

In FIG. 8, a one-dot chain line represents the transmission efficiencyof the drive system 10 placed in the mode 3 (HV-1), while a solid linerepresents the transmission efficiency in the mode 4 (HV-2). Asindicated in FIG. 8, the transmission efficiency of the drive system 10in the mode 3 (HV-1) has a maximum value at a speed ratio value γ1. Atthis speed ratio value γ1, the operating speed of the first electricmotor MG1 (rotating speed of the sun gear S1) is zero, and an amount ofan electric energy transmitted through the electric path is zero duringgeneration of the reaction force, so that the drive force is onlymechanically transmitted from the engine 12 and the second electricmotor MG2 to the output gear 30, at an operating point corresponding tothe speed ratio value γ1. This operating point at which the transmissionefficiency is maximum while the amount of the electric energytransmitted through the electric path is zero will be hereinafterreferred to as a “mechanical point (mechanical transmission point)”. Thespeed ratio value γ1 is lower than “1”, that is, a speed ratio on anoverdrive side, and will be hereinafter referred to as a “firstmechanical transmission speed ratio value γ1”. As indicated in FIG. 8,the transmission efficiency in the mode 3 gradually decreases with anincrease of the speed ratio from the first mechanical transmission speedratio value γ1 toward the low-gear side, and abruptly decreases with adecrease of the speed ratio from the first mechanical transmission speedratio value γ1 toward the high-gear side.

In the mode 4 (HV-2) of the drive system 10, the gear ratios ρ1 and ρ2of the first planetary gear set 14 and second planetary gear set 16having the four rotary elements in the engaged state of the clutch CLare determined such that the operating speeds of the first electricmotor MG1 and second electric motor MG2 are indicated at respectivedifferent positions along the horizontal axis of the collinear chart ofFIG. 6, so that the transmission efficiency in the mode 4 has a maximumvalue at a mechanical point at a speed ratio value γ2, as well as at thespeed ratio value γ1, as indicated in FIG. 8. Namely, in the mode 4, therotating speed of the first electric motor MG1 is zero at the firstmechanical transmission speed ratio value γ1 at which the amount of theelectric energy transmitted through the electric path is zero duringgeneration of the reaction force by the first electric motor MG1, whilethe rotating speed of the second electric motor MG2 is zero at the speedratio value γ2 at which the amount of the electric energy transmittedthrough the electric path is zero during generation of the reactionforce by the second electric motor MG2. The speed ratio value γ2 will behereinafter referred to as a “second mechanical transmission speed ratiovalue γ2”. This second mechanical transmission speed ratio value γ2 issmaller than the first mechanical transmission speed ratio value γ1. Inthe mode 4, the drive system 10 has the mechanical point located on thehigh-gear side of the mechanical point in the mode 3.

As indicated in FIG. 8, the transmission efficiency in the mode 4 moreabruptly decreases with an increase of the speed ratio on a low-gearside of the first mechanical transmission speed ratio value γ1, than thetransmission efficiency in the mode 3. In a region of the speed ratiobetween the first mechanical transmission speed ratio value γ1 andsecond mechanical transmission speed ratio value γ2, the transmissionefficiency in the mode 4 changes along a concave curve. In this region,the transmission efficiency in the mode 4 is almost equal to or higherthan that in the mode 3 . The transmission efficiency in the mode 4decreases with a decrease of the speed ratio from the second mechanicaltransmission speed ratio value γ2 toward the high-gear side, but ishigher than that in the mode 3. That is, the drive system placed in themode 4 has not only the first mechanical transmission speed ratio valueγ1, but also the second mechanical transmission speed ratio value γ2 onthe high-gear side of the first mechanical transmission speed ratiovalue γ1, so that the transmission efficiency of the drive system can beimproved in high-gear positions having comparatively low speed ratiovalues. Thus, a fuel economy during running of the vehicle at arelatively high speed is improved owing to an improvement of thetransmission efficiency.

As described above referring to FIG. 8, the transmission efficiency ofthe drive system 10 during a hybrid running of the vehicle with anoperation of the engine 12 used as the vehicle drive power source andoperations of the first and second electric motors MG1 and MG2 as neededto generate a vehicle drive force and/or an electric energy can beimproved by adequately switching the vehicle drive mode between the mode3 (HV-1) and mode 4 (HV-2). For instance, the mode 3 is established inlow-gear positions having speed ratio values lower than the firstmechanical transmission speed ratio value γ1, while the mode 4 isestablished in high-gear positions having speed ratio values higher thanthe first mechanical transmission speed ratio value γ1, so that thetransmission efficiency can be improved over a wide range of the speedratio covering the low-gear region and the high-gear region.

The drive mode HV-3 indicated in FIG. 3 corresponds to the mode 5 (drivemode 5) of the drive system 10, which is preferably the hybrid drivemode in which the engine 12 is operated as the vehicle drive powersource while the first electric motor MG1 is operated as needed togenerate a vehicle drive force and/or an electric energy. In this mode5, the engine 12 and first electric motor MG1 may be operated togenerate a vehicle drive force, with the second electric motor MG2 beingdisconnected from a drive system. FIG. 7 is the collinear chartcorresponding to this mode 5. Described by reference to this collinearchart, the carrier C1 of the first planetary gear set 14 and the carrierC2 of the second planetary gear set 16 are rotatable relative to eachother in the released state of the clutch CL. In the released state ofthe brake BK, the carrier C2 of the second planetary gear set 16 isrotatable relative to the stationary member in the form of the housing26. In this arrangement, the second electric motor MG2 can be held atrest while it is disconnected from the drive system (power transmittingpath).

In the mode 3 in which the brake BK is placed in the engaged state, thesecond electric motor MG2 is kept in an operated state together with arotary motion of the output gear 30 (ring gear R2) during running of thevehicle. In this operating state, the operating speed of the secondelectric motor MG2 may reach an upper limit value (upper limit) duringrunning of the vehicle at a comparatively high speed, or a rotary motionof the ring gear R2 at a high speed is transmitted to the sun gear S2.In this respect, it is not necessarily desirable to keep the secondelectric motor MG2 in the operated state during running of the vehicleat a comparatively high speed, from the standpoint of the operatingefficiency. In the mode 5, on the other hand, the engine 12 and thefirst electric motor MG1 may be operated to generate the vehicle driveforce during running of the vehicle at the comparatively high speed,while the second electric motor MG2 is disconnected from the drivesystem, so that it is possible to reduce a power loss due to dragging ofthe unnecessarily operated second electric motor MG2, and to eliminate alimitation of the highest vehicle running speed corresponding to thepermissible highest operating speed (upper limit of the operating speed)of the second electric motor MG2.

It will be understood from the foregoing description, the drive system10 is selectively placed in one of the three hybrid drive modes in whichthe engine 12 is operated as the vehicle drive power source, namely, inone of the drive mode HV-1 (mode 3), drive mode HV-2 (mode 4) and drivemode HV-3 (mode 5), which are selectively established by respectivecombinations of the engaged and released states of the clutch CL andbrake BK. Accordingly, the transmission efficiency can be improved toimprove the fuel economy of the vehicle, by selectively establishing oneof the three hybrid drive modes according to the vehicle running speedand the speed ratio, in which the transmission efficiency is thehighest.

FIG. 9 is the functional block diagram for explaining major controlfunctions of the electronic control device 40. A drive mode determiningportion 70, shown in FIG. 9, is configured to determine one of the drivemodes of the drive system 10 to be established. The drive modedetermining portion 70 is basically configured to select one of themodes 1-5 described above by reference to FIG. 3, according to apredetermined relationship and on the basis of the accelerator pedaloperation amount A_(CC) detected by the accelerator pedal operationamount sensor 42, the vehicle running speed V corresponding to theoutput speed N_(OUT) detected by the output speed sensor 50, and thebattery SOC detected by the battery SOC sensor 54, for example.Preferably, the drive mode determining portion 70 selects one of thehybrid drive modes in the form of the modes 3-5 in which the engine 12is operated as the vehicle drive power source, when the battery SOCdetected by the battery SOC sensor 54 is smaller than a predeterminedthreshold value. For instance, the drive mode determining portion 70selects the EV drive mode in the form of the mode 1 or 2 in which theengine 12 is held at rest, when the battery SOC detected by the batterySOC sensor 54 is not smaller than the above-indicated threshold value.Upon starting of the hybrid vehicle, namely, upon a releasing action ofthe brake pedal (not shown) (from the operated position to thenon-operated position) when the vehicle running speed V corresponding tothe output speed N_(OUT) detected by the output speed sensor 50 is zerowhile the battery SOC detected by the battery SOC sensor 54 is notsmaller than the above-indicated threshold value, for instance, thedrive mode determining portion 70 selects the EV drive mode in the formof the mode 1 in which the engine 12 is held at rest while the firstelectric motor MG1 is primarily used as the vehicle drive power source.The drive mode determining portion 70 selects one of the drive modesaccording to the specific running state of the hybrid vehicle providedwith the drive system 10, so as to improve the transmission efficiencyand the fuel economy of the engine 12.

A clutch engagement control portion 72 is configured to control theoperating state of the clutch CL through the hydraulic control unit 60.For instance, the clutch engagement control portion 72 controls anoutput hydraulic pressure of an electromagnetic control valve providedin the hydraulic control unit 60 to control the clutch CL, so as toplace the clutch CL in an engaged state or a released state. A brakeengagement control portion 74 is configured to control the operatingstate of the brake BK through the hydraulic control unit 60. Forinstance, the brake engagement control portion 74 controls an outputhydraulic pressure of an electromagnetic control valve provided in thehydraulic control unit 60 to control the brake BK, so as to place thebrake BK in an engaged state or a released state. The clutch engagementcontrol portion 72 and the brake engagement control portion 74 arebasically configured to control the operating states of the clutch CLand the brake BK to establish the drive mode selected by the drive modedetermining portion 70. Namely, the clutch and brake engagement controlportions 72 and 74 establish one of the combinations of the operatingstates of the clutch CL and the brake BK indicated in FIG. 3, whichcorresponds to one of the modes 1-5 to be established.

An engine starting determining portion 76 is configured to determinewhether the engine 12 which has been held at rest is required to bestarted. Preferably, the engine starting determining portion 76 makesthe determination as to whether the engine 12 is required to be started,depending upon the drive mode selected by the drive mode determiningportion 70. Namely, the engine starting determining portion 76determines that the engine 12 is required to be started, when the drivemode determining portion 70 has determined that the drive mode should beswitched from one of the EV drive modes in which the at least one of thefirst and second electric motors MG1 and MG2 is operated as the vehicledrive power source while the engine 12 is held at rest, to one of thehybrid drive modes in which the engine 12 is operated. Described morespecifically, the engine starting determining portion 76 determines thatthe engine 12 is required to be started, when the drive mode determiningportion 70 has determined that the drive mode should be switched fromthe mode 1 (EV-1) or the mode 2 (EV-2) shown in FIG. 3 to the mode 3(HV-1), the mode 4 (HV-2) or the mode 5 (HV-3). When the engine startingdetermining portion 76 has determined that the engine 12 is required tobe started, the rotating speed of the input shaft 28 (rotating speed ofthe carrier C1) corresponding to the rotating speed of the crankshaft ofthe engine 12 is raised by torque of at least one of the first andsecond electric motors MG1 and MG2, and the operation of the engine 12by itself is initiated under the control of the engine control device56.

A reverse drive force estimating portion 78 is configured to estimate(determine) whether the operator of the hybrid vehicle is unlikely tofeel generation of a reverse drive force upon starting of the engine 12which has been held at rest. This reverse drive force is a drive force(acceleration) in the direction opposite to the direction of running(e.g., forward running direction) of the hybrid vehicle provided withthe drive system 10, and is felt by the operator as deceleration of thehybrid vehicle. That is, or in other words, the reverse drive forceestimating portion 78 determines whether the operator is unlikely tofeel the deceleration of the hybrid vehicle during starting of theengine 12 which has been held at rest. To make this determination, thereverse drive force estimating portion 78 includes a drive forcedetermining portion 80, a brake operation determining portion 82 and aP-position determining portion 84. Each of the functions of thesedetermining portions 80, 82 and 84 will be described.

The above-indicated drive force determining portion 80 is configured todetermine whether a drive force of the hybrid vehicle generated duringstarting of the engine 12 which has been held at rest is equal to orlarger than a predetermined threshold value P_(bo). Preferably, thedrive force determining portion 80 determines whether a required driveforce (an amount of drive force required by the operator) calculated onthe basis of the accelerator pedal operation amount A_(CC) and thevehicle running speed V, for example, is equal to or larger than thepredetermined threshold value P_(bo). If an affirmative determination isobtained by the drive force determining portion 80, that is, if thegenerated drive force is equal to or larger than the predeterminedthreshold value P_(bo), the reverse drive force estimating portion 78makes an affirmative determination. Namely, the reverse drive forceestimating portion 78 makes the determination that the operator isunlikely to feel the generation of the reverse drive force upon startingof the engine 12 which has been held at rest. If a negativedetermination is obtained by the drive force determining portion 80,that is, if the generated drive force is smaller than the predeterminedthreshold value P_(bo), the reverse drive force estimating portion 78makes a negative determination. Namely, the reverse drive forceestimating portion 78 makes the determination that the operator is notunlikely (is likely) to feel the generation of the reverse drive forceupon starting of the engine 12 which has been held at rest.

The above-indicated brake operation determining portion 82 is configuredto determine whether a vehicle braking operation is performed, that is,whether the output signal of the brake sensor 55 indicates the vehiclebraking operation (an operated position of the manually operated brakingmember). Preferably, the brake operation determining portion 82determines whether the hybrid vehicle is held stationary with thebraking operation. Namely, the brake operation determining portion 82determines whether the output signal of the brake sensor 55 indicatesthe vehicle braking operation (the operated position of the manuallyoperated braking member) while at the same time the output speed N_(OUT)(vehicle running speed V) detected by the output speed sensor 50 iszero. If an affirmative determination is obtained by the brake operationdetermining portion 82, that is, if the braking operation is performed(the vehicle is held stationary with the braking operation), the reversedrive force estimating portion 78 makes the affirmative determination.Namely, the reverse drive force estimating portion 78 makes thedetermination that the operator is unlikely to feel the generation ofthe reverse drive force upon starting of the engine 12 which has beenheld at rest. If a negative determination is obtained by the brakeoperation determining portion 82, that is, if the braking operation isnot performed (the vehicle is not held stationary with the brakingoperation), the reverse drive force estimating portion 78 makes thenegative determination. Namely, the reverse drive force estimatingportion 78 makes the determination that the operator is not unlikely (islikely) to feel the generation of the reverse drive force upon startingof the engine 12 which has been held at rest.

The above-indicated P-position determining portion 84 is configured todetermine whether the drive system 10 is placed in a parking state,namely, whether the presently selected operating position P_(S) of themanually operated shifting device detected by the shift position sensor52 is the “P” position. If an affirmative determination is obtained bythe P-position determining portion 84, that is, if the presentlyselected operating position P_(S) is the “P” position, the reverse driveforce estimating portion 78 makes the affirmative determination. Namely,the reverse drive force estimating portion 78 makes the determinationthat the operator is unlikely to feel the generation of the reversedrive force upon starting of the engine 12 which has been held at rest.If a negative determination is obtained by the P-position determiningportion 84, that is, if the presently selected operating position P_(S)is not the “P” position, the reverse drive force estimating portion 78makes the negative determination. Namely, the reverse drive forceestimating portion 78 makes the determination that the operator is notunlikely (is likely) to feel the generation of the reverse drive forceupon starting of the engine 12 which has been held at rest. Preferably,the reverse drive force estimating portion 78 is configured to make theaffirmative determination if at least one of the drive force determiningportion 80, the brake operation determining portion 82 and theP-position determining portion 84 has made the affirmativedetermination, and make the negative determination if all of thosedetermining portions 80, 82 and 84 have made the negativedeterminations.

An engine starting engagement control portion 86 is configured toimplement an engagement control for releasing at least one of the clutchCL and the brake BK, when the engine 12 is started in the drive mode inwhich the engine 12 is held at rest while the clutch CL and the brake BKare both placed in the engaged state, that is, in the mode 2 (EV-2).That is, the engine starting engagement control portion 86 implementsthe engagement control to release at least one of the clutch CL and thebrake BK, when the engine starting determining portion 76 has determinedthat the engine 12 is required to be started, as a result of thedetermination by the drive mode determining portion 70 that the drivemode should be switched while the mode 2 is established. Preferably, theengine starting engagement control portion 86 implements the engagementcontrol to release either one of the clutch CL and the brake BK (clutchCL or brake BK) when the engine 12 is started while the mode 2 isestablished.

As described above by reference to the collinear chart of FIG. 5, thecarrier C1 of the first planetary gear set 14 and the carrier C2 of thesecond planetary gear set 16 are not rotatable relative to each otherwhen the drive system 10 is placed in the mode 2 in which the clutch CLis placed in the engaged state. Further, the brake BK is placed in theengaged state in the mode 2, so that the carrier C2 of the secondplanetary gear set 16, and the carrier C1 of the first planetary gearset 14 connected to the carrier C2 are connected (fixed) to thestationary member in the form of the housing 26, whereby the rotatingspeeds of the carriers C2 and C1 are zero. Accordingly, the rotatingspeed of the input shaft 28 (rotating speed of the carrier C1)corresponding to the operating speed of the engine 12 cannot be raised,so that the engine 12 cannot be started. Therefore, upon starting of theengine 12 while the mode 2 is established, the engine startingengagement control portion 86 implements the engagement control torelease at least one of the clutch CL and the brake BK, for permittingthe operating speed of the engine 12 to be raised, to permit the engine12 to be started.

The engine starting engagement control portion 86 is preferablyconfigured to release one of the clutch CL and the brake BK uponstarting of the engine 12 in the mode 2, and to select one of the clutchCL and the brake BK which is to be released, depending upon the driveforce of the hybrid vehicle generated during starting of the engine 12.Namely, the engine starting engagement control portion 86 selectivelyreleases one of the clutch CL and the brake BK depending upon a resultof the determination by the drive force determining portion 80. If thedrive force determining portion 80 has made the affirmativedetermination, that is, the determination that the generated drive forceis equal to or larger than the predetermined threshold value P_(bo), forinstance, the engine starting engagement control portion 86 releases thebrake BK. If the drive force determining portion 80 has made thenegative determination, that is, the determination that the generateddrive force is smaller than the predetermined threshold value P_(bo), onthe other hand, the engine starting engagement control portion 86releases the clutch CL.

The engine starting engagement control portion 86 is preferablyconfigured to release one of the clutch CL and the brake BK uponstarting of the engine 12 while the mode 2 is established, and to selectone of the clutch CL and the brake BK to be released, depending uponwhether the operator is unlikely to feel the generation of the reversedrive force upon starting of the engine 12. Namely, the engine startingengagement control portion 86 selectively releases one of the clutch CLand the brake BK, depending upon a result of the determination by thereverse drive force estimating portion 78. If the reverse drive forceestimating portion 78 has made the affirmative determination, that is,the determination that the operator is unlikely to feel the generationof the reverse drive force due to starting of the engine, for instance,the engine starting engagement control portion 86 releases the brake BK.If the reverse drive force estimating portion 78 has made the negativedetermination, that is, the determination that the operator is notunlikely (is likely) to feel the generation of the reverse drive forcedue to starting of the engine, on the other hand, the engine startingengagement control portion 86 releases the clutch CL.

In other words, the engine starting engagement control portion 86 ispreferably configured to selectively release one of the clutch CL andthe brake BK upon starting of the engine 12 while the mode 2 isestablished, depending upon a result of the determination by the brakeoperation determining portion 82. If the brake operation determiningportion 82 has made the affirmative determination, that is, thedetermination that the braking operation is performed (preferably, thehybrid vehicle is held stationary with the braking operation), forinstance, the engine starting engagement control portion 86 releases thebrake BK. If the brake operation determining portion 82 has made thenegative determination, that is, the determination that the brakingoperation is not performed (preferably, the hybrid vehicle is not heldstationary with the braking operation), on the other hand, the enginestarting engagement control portion 86 releases the clutch CL.

In other words, the engine starting engagement control portion 86 ispreferably configured to selectively release one of the clutch CL andthe brake BK upon starting of the engine 12 while the mode 2 isestablished, depending upon a result of the determination by theP-position determining portion 84. If the P-position determining portion84 has made the affirmative determination, that is, the determinationthat the presently selected operating position P_(S) of the manuallyoperated shifting device detected by the shift position sensor 52 is the“P” position, for instance, the engine starting engagement controlportion 86 releases the brake BK. If the P-position determining portion84 has made the negative determination, that is, the determination thatthe presently selected operating position P_(S) of the manually operatedshifting device detected by the shift position sensor 52 is not the “P”position, on the other hand, the engine starting engagement controlportion 86 releases the clutch CL.

If the drive force of the hybrid vehicle provided with the drive system10 is comparatively large (for instance, equal to or larger than thepredetermined threshold value P_(bo)), or if the hybrid vehicle is heldstationary with the braking operation, or if the presently selectedoperating position P_(S) of the manually operated shifting device is the“P” position, the operator is unlikely to feel the generation of areverse drive force (deceleration) upon starting of the engine 12 whichhas been held at rest. The drive system 10 is configured such that whereeither one of the clutch CL and the brake BK is released upon startingof the engine while the clutch CL and the brake BK are both placed inthe engaged state, the generated reverse drive force is larger when thebrake BK is released than when the clutch CL is released. Namely,although it is generally impossible to reduce the reverse drive force byreleasing the brake BK upon starting of the engine 12, the operator isunlikely to feel uneasy about a releasing action of the brake BK, in adriving state of the hybrid vehicle in which the operator is unlikely tofeel the generation of the reverse drive force. In a driving state ofthe hybrid vehicle in which the operator is likely to feel thegeneration of the reverse drive force, on the other hand, there is arisk that the operator feels uneasy about the releasing action of thebrake BK to start the engine 12. Accordingly, the brake BK is releasedwhere the operator is unlikely to feel the generation of the reversedrive force upon starting of the engine 12, but the clutch CL isreleased where the operator is not unlikely to feel the generation ofthe reverse drive force, so that the uneasiness felt by the operatorupon starting of the engine 12 in the mode 2 can be effectively reduced.

FIG. 10 is the flow chart for explaining a major portion of an exampleof an engine starting control implemented by the electronic controldevice 40. The engine starting control is repeatedly implemented with apredetermined cycle time.

The engine starting control is initiated with step S1 (“step” beinghereinafter omitted), to determine whether the engine 12 which has beenheld at rest is required to be started as a result of the determinationthat the drive mode should be switched from the EV drive mode to thehybrid drive mode, for example. If a negative determination is obtainedin S1, the present routine is terminated. If an affirmativedetermination is obtained in S1, the control flow goes to S2 todetermine whether the drive system 10 is presently placed in the mode 2(EV-2) in which the engine 12 is held at rest while the clutch CL andthe brake BK are both placed in the engaged state. If a negativedetermination is obtained in S2, the present routine is terminated. Ifan affirmative determination is obtained in S2, the control flow goes toS3 to generate a command to release either one of the clutch CL and thebrake BK. Then, the control flow goes to S4 to determine whether thereleasing action (hydraulic pressure control for the releasing action)of the clutch CL or the brake BK is completed. If a negativedetermination is obtained in S4, the control flow goes back to S3. If anaffirmative determination is obtained in S4, the control flow goes to S5to implement the control to start the engine 12, and the present routineis terminated.

FIG. 11 is the flow chart for explaining a major portion of anotherexample of the engine starting control implemented by the electroniccontrol device 40. This drive mode switching control is repeatedlyimplemented with a predetermined cycle time. In a description forcontrol shown in FIG. 11, the same reference signs are used forcorresponding steps as those in the control shown in FIG. 10, and thepractical descriptions are omitted.

If an affirmative determination is obtained in S2 in the engine startingcontrol illustrated in FIG. 11, namely, if the drive system 10 is placedin the mode 2 (EV-2) in which the engine 12 is held at rest while theclutch CL and the brake BK are both placed in the engaged state, thecontrol flow goes to S6 to determine whether the operator is unlikely tofeel the generation of the reverse drive force upon starting of theengine 12. For example, this determination is made by determiningwhether the required vehicle drive force calculated on the basis of theaccelerator pedal operation amount A_(CC) and the vehicle running speedV is equal to or larger than the predetermined threshold value P_(bo),whether the hybrid vehicle is held stationary with the brakingoperation, or whether the presently selected operating position P_(S) ofthe manually operated shifting device is the “P” position. If anaffirmative determination is obtained in S6, that is, if it isdetermined that the operator is unlikely to feel the generation of thereverse drive force upon starting of the engine 12, the control flowgoes to S7 to generate a command to release the brake BK. Then, thecontrol flow goes to S8 to determine whether the releasing action(hydraulic control for the releasing action) of the brake BK iscompleted. If a negative determination is obtained in S8, the controlflow goes back to S7. If an affirmative determination is obtained in S8,the control flow goes to S5. If a negative determination is obtained inS6, that is, if it is determined that the operator is not unlikely (islikely) to feel the generation of the reverse drive force upon startingof the engine 12, the control flow goes to S9 to generate a command torelease the clutch CL. Then, the control flow goes to S10 to determinewhether the releasing action (hydraulic control for the releasingaction) of the clutch CL is completed. If a negative determination isobtained in S10, the control flow goes back to S9. If an affirmativedetermination is obtained in S10, the control flow goes to S5.

It will be understood from the foregoing description that S2 correspondsto the operation of the drive mode determining portion 70, and S3, S4,S9 and S10 correspond to the operation of the clutch engagement controlportion 72, while S3, S4, S7 and S8 correspond to the operation of thebrake engagement control portion 74, and that S1 corresponds to theoperation of the engine starting determining portion 76, and S6corresponds to the operation of the reverse drive force estimatingportion 78 (drive force determining portion 80), while S3, S4 and S7-S10correspond to the operation of the engine starting engagement controlportion 86.

Other preferred embodiments of the present invention will be describedin detail by reference to the drawings. In the following description,the same reference signs will be used to identify the same elements inthe different embodiments, which will not be described redundantly.

Second Embodiment

FIGS. 12-17 are the schematic views for explaining arrangements ofrespective hybrid vehicle drive systems 100, 110, 120, 130, 140 and 150according to other preferred modes of this invention. The hybrid vehicledrive control device of the present invention is also applicable todrive systems such as the drive system 100 shown in FIG. 12 and thedrive system 110 shown in FIG. 13, which have respective differentarrangements of the first electric motor MG1, first planetary gear set14, second electric motor MG2, second planetary gear set 16, clutch CLand brake BK in the direction of the center axis CE. The present hybridvehicle drive control device is also applicable to drive systems such asthe drive system 120 shown in FIG. 14, which have a one-way clutch OWCdisposed between the carrier C2 of the second planetary gear set 16 andthe stationary member in the form of the housing 26, in parallel withthe brake BK, such that the one-way clutch OWC permits a rotary motionof the carrier C2 relative to the housing 26 in one of oppositedirections and inhibits a rotary motion of the carrier C2 in the otherdirection. The present hybrid vehicle drive control device is furtherapplicable to drive systems such as the drive system 130 shown in FIG.15, the drive system 140 shown in FIG. 16 and the drive system 150 shownin FIG. 17, which is provided with a second differential mechanism inthe form of a second planetary gear set 16′ of a double-pinion type, inplace of the second planetary gear set 16 of a single-pinion type. Thissecond planetary gear set 16′ is provided with rotary elements(elements) consisting of: a first rotary element in the form of a sungear S2′; a second rotary element in the form of a carrier C2′supporting a plurality of pinion gears P2′ meshing with each other suchthat each pinion gear P2′ is rotatable about its axis and the axis ofthe planetary gear set; and a third rotary element in the form of a ringgear R2′ meshing with the sun gear S2′ through the pinion gears P2′.

Third Embodiment

FIGS. 18-20 are the collinear charts for explaining arrangements andoperations of hybrid vehicle drive systems 160, 170 and 180 according tofurther preferred modes of this invention in place of the drive system10. In FIGS. 18-20, the relative rotating speeds of the sun gear S1carrier C1 and ring gear R1 of the first planetary gear set 14 arerepresented by the solid line L1, while the relative rotating speeds ofthe sun gear S2, carrier C2 and ring gear R2 of the second planetarygear set 16 are represented by the broken line L2, as in FIGS. 4-7. Inthe drive system 160 for the hybrid vehicle shown in FIG. 18, the sungear S1, carrier C1 and ring gear R1 of the first planetary gear set 14are respectively connected to the first electric motor MG1, engine 12and second electric motor MG2, while the sun gear S2, carrier C2 andring gear R2 of the second planetary gear set 16 are respectivelyconnected to the second electric motor MG2 and output gear 30, and tothe housing 26 through the brake BK. The sun gear Si and the ring gearR2 are selectively connected to each other through the clutch CL. Thering gear R1 and the sun gear S2 are connected to each other. In thedrive system 170 for the hybrid vehicle shown in FIG. 19, the sun gearS1, carrier C1 and ring gear R1 of the first planetary gear set 14 arerespectively connected to the first electric motor MG1, output gear 30and engine 12, while the sun gear S2, carrier C2 and ring gear R2 of thesecond planetary gear set 16 are respectively connected to the secondelectric motor MG2 and output gear 30, and to the housing 26 through thebrake BK. The sun gear S1 and the ring gear R2 are selectively connectedto each other through the clutch CL. The carriers Cl and C2 areconnected to each other. In the drive system 180 for the hybrid vehicleshown in FIG. 20, the sun gear S1, carrier Cl and ring gear R1 of thefirst planetary gear set 14 are respectively connected to the firstelectric motor MG1, output gear 30 and engine 12, while the sun gear S2,carrier C2 and ring gear R2 of the second planetary gear set 16 arerespectively connected to the second electric motor MG2, to the housing26 through the brake BK, and to the output gear 30. The ring gear R1 andthe carrier C2 are selectively connected to each other through theclutch CL. The carrier Cl and ring gear R2 are connected to each other.

The drive systems for the hybrid vehicle shown in FIGS. 18-20 areidentical with each other in that each of these drive systems for thehybrid vehicle is provided with the first differential mechanism in theform of the first planetary gear set 14 and the second differentialmechanism in the form of the second planetary gear set 16, 16′, whichhave four rotary elements (whose relative rotating speeds arerepresented) in the collinear chart, and is further provided with thefirst electric motor MG1, second electric motor MG2, engine 12 andoutput rotary member (output gear 30) which are connected to therespective four rotary elements, and wherein one of the four rotaryelements is constituted by the rotary element of the first planetarygear set 14 and the rotary element of the second planetary gear set 16,16′ which are selectively connected to each other through the clutch CL,and the rotary element of the second planetary gear set 16, 16′selectively connected to the rotary element of the first planetary gearset 14 through the clutch CL is selectively fixed to the housing 26 asthe stationary member through the brake BK, as in the drive system forthe hybrid vehicle shown in FIGS. 4-7. Namely, the hybrid vehicle drivecontrol device of the present invention described above by reference toFIG. 9 and the other figures is also applicable to the drive systemsshown in FIGS. 18-20.

As described above, the illustrated embodiments described above areconfigured such that the hybrid vehicle is provided with: the firstdifferential mechanism in the form of the first planetary gear set 14and the second differential mechanism in the form of the secondplanetary gear set 16, 16′ which have the four rotary elements as awhole when the clutch CL is placed in the engaged state (and thus thefirst planetary gear set 14 and the second planetary gear set 16, 16′are represented as the four rotary elements in the collinear charts suchas FIGS. 4-7); and the engine 12, the first electric motor MG1, thesecond electric motor MG2 and the output gear 30 which are respectivelyconnected to the four rotary elements. One of the four rotary elementsis constituted by the rotary element of the first differential mechanismand the rotary element of the second differential mechanism which areselectively connected to each other through the clutch CL, and one ofthe rotary elements of the first and second differential mechanismswhich are selectively connected to each other through the clutch CL isselectively fixed to the stationary member in the form of the housing 26through the brake BK. The drive control device is configured to releaseat least one of the clutch CL and the brake BK when the engine 12 isstarted in the mode 2 (EV-2) in which the engine 12 is held at restwhile the clutch CL and the brake BK are both placed in the engagedstate. Accordingly, it is possible to effectively reduce the generationof the reverse drive force upon starting of the engine which has beenheld at rest. Namely, the illustrated embodiments provide a hybridvehicle drive control device in the form of the electronic controldevice 40 which permits reduction of uneasiness felt by the hybridvehicle operator upon starting of the engine which has been held atrest.

The illustrated embodiments are further configured to release one of theclutch CL and the brake BK when the engine 12 is started in the mode 2,and to select the above-indicated one of the clutch CL and the brake BKwhich is to be released, depending upon the drive force generated todrive the hybrid vehicle during starting of the engine 12. Accordingly,it is possible to effectively and practically reduce the generation ofthe reverse drive force upon starting of the engine which has been heldat rest.

The illustrated embodiments are also configured to release one of theclutch CL and the brake BK when the engine 12 is started in the mode 2,and to release the brake BK when it is determined that the operator ofthe hybrid vehicle is unlikely to feel generation of a reverse driveforce due to starting of the engine, and release the clutch CL when itis determined that the operator of the hybrid vehicle is not unlikely tofeel the generation of the reverse drive force. Accordingly, it ispossible to effectively and practically reduce the generation of thereverse drive force upon starting of the engine which has been held atrest.

The illustrated embodiments are further configured to release the brakeBK when the generated drive force is equal to or larger than thepredetermined threshold value P_(bo), and to release the clutch CL whenthe generated drive force is smaller than the predetermined thresholdvalue P_(bo). Accordingly, it is possible to effectively and practicallyreduce the generation of the reverse drive force upon starting of theengine which has been held at rest.

The illustrated embodiments are also configured such that the firstplanetary gear set 14 is provided with a first rotary element in theform of the sun gear S1 connected to the first electric motor MG1, asecond rotary element in the form of the carrier C1 connected to theengine 12, and a third rotary element in the form of the ring gear R1connected to the output gear 30, while the second planetary gear set 16(16′) is provided with a first rotary element in the form of the sungear S2 (S2′) connected to the second electric motor MG2, a secondrotary element in the form of the carrier C2 (C2′), and a third rotaryelement in the form of the ring gear R2 (R2′), one of the carrier C2(C2′) and the ring gear R2 (R2′) being connected to the ring gear R1 ofthe first planetary gear set 14. The clutch CL is configured toselectively connect the carrier C1 of the first planetary gear set 14and the other of the carrier C2 (C2′) and the ring gear R2 (R2′) whichis not connected to the ring gear R1, to each other, while the brake BKis configured to selectively fix the other of the carrier C2 (C2′) andthe ring gear R2 (R2′) which is not connected to the ring gear R1, to astationary member in the form of the housing 26. Accordingly, it ispossible to effectively reduce the generation of the reverse drive forceupon starting of the engine which has been held at rest.

While the preferred embodiments of this invention have been described byreference to the drawings, it is to be understood that the invention isnot limited to the details of the illustrated embodiments, but may beembodied with various changes which may occur without departing from thespirit of the invention.

NOMENCLATURE OF REFERENCE SIGNS

-   10, 100, 110, 120, 130, 140, 150, 160, 170, 180: Hybrid vehicle    drive system-   12: Engine 14: First planetary gear set (First differential    mechanism)-   16, 16′: Second planetary gear set (Second differential mechanism)-   18, 22: Stator 20, 24: Rotor 26: Housing (Stationary member)-   28: Input shaft 30: Output gear (Output rotary member)-   32: Oil pump 40: Electronic control device (Drive control device)-   42: Accelerator pedal operation amount sensor 44: Engine speed    sensor-   46: MG1 speed sensor 48: MG2 speed sensor 50: Output speed sensor-   52: Shift position sensor 54: Battery SOC sensor 55: Brake sensor-   56: Engine control device 58: Inverter 60: Hydraulic control unit-   70: Drive mode determining portion-   72: Clutch engagement control portion-   74: Brake engagement control portion-   76: Engine starting determining portion-   78: Reverse drive force estimating portion-   80: Drive force determining portion-   82: Brake operation determining portion-   84: P-position determining portion-   86: Engine starting engagement control portion-   BK: Brake CL: Clutch C1, C2, C2′: Carrier (Second rotary element)-   MG1: First electric motor MG2: Second electric motor-   OWC: One-way clutch P1, P2, P2′: Pinion gear-   R1, R2, R2′: Ring gear (Third rotary element)-   S1, S2, S2′: Sun gear (First rotary element)

The invention claimed is:
 1. A drive control device for a hybrid vehicleprovided with: a differential device which includes a first differentialmechanism and a second differential mechanism and which has four rotaryelements; and an engine, a first electric motor, a second electric motorand an output rotary member which are connected to said respective fourrotary elements, and wherein one of said four rotary elements isconstituted by a rotary component of said first differential mechanismand a rotary component of said second differential mechanism which areselectively connected to each other through a clutch, and one of therotary components of said first and second differential mechanisms whichare selectively connected to each other through said clutch isselectively fixed to a stationary member through a brake, said drivecontrol device releasing one of said clutch and said brake when saidengine is started in a drive mode in which said engine is held at restwhile said clutch and said brake are both placed in an engaged state,selecting said one of said clutch and said brake which is to bereleased, depending upon a drive force generated by at least one of thefirst electric motor and the second electric motor to drive said hybridvehicle during starting of said engine.
 2. The drive control deviceaccording to claim 1, wherein one of said clutch and said brake isreleased when said engine is started in said drive mode, and said brakeis released when at least one of (i) a determination that said driveforce is equal to or larger than a predetermined threshold value, (ii) adetermination that the hybrid vehicle is held stationary with a brakingoperation, and (iii) a determination that a manually operated shiftingdevice is place in a parking position, is made.
 3. The drive controldevice according to claim 1, wherein said brake is released when saiddrive force is equal to or larger than a predetermined threshold value.4. The drive control device according to claim 1, wherein said clutch isreleased when said drive force is smaller than a predetermined thresholdvalue.
 5. The drive control device according to claim 1, wherein saidfirst differential mechanism is provided with: a first rotary elementconnected to said first electric motor, a second rotary elementconnected to said engine, and a third rotary element connected to saidoutput rotary member, and said second differential mechanism is providedwith: a first rotary element connected to said second electric motor, asecond rotary element, and a third rotary element, one of the second andthird rotary elements of the second differential mechanism beingconnected to the third rotary element of said first differentialmechanism, and wherein said clutch is configured to selectively connectthe second rotary element of said first differential mechanism, and theother of the second and third rotary elements of said seconddifferential mechanism which is not connected to the third rotaryelement of said first differential mechanism, to each other, while saidbrake is configured to selectively fix the other of the second and thirdrotary elements of said second differential mechanism which is notconnected to the third rotary element of said first differentialmechanism, to the stationary member.